Osseous Tissue

Osseous Tissue

Osseous Connective Tissue

  • Approximately 206 bones in the human body.
  • Main Functions of Bone:
    • Support: Provides a rigid framework for the body.
    • Protection: Shields vital organs (e.g., brain, heart).
    • Movement: Serves as levers that muscles pull on, enabling locomotion.
    • Electrolyte/pH balance: Helps maintain homeostasis through the regulation of minerals such as calcium and phosphorus.
    • Blood Formation: Hematopoiesis, the production of blood cells, occurs in the red bone marrow.

Classes of Bones

  • Four Classes of Bones:
    • Flat Bones:
    • Characteristics: Thin, flattened, and usually a bit curved.
    • Examples: Cranial bones, sternum, scapula, ribs, and hip bones.
    • Long Bones:
    • Characteristics: Longer than they are wide, with a shaft and two ends.
    • Examples: Bones of the limbs (e.g., femur, humerus), metacarpals, tarsals, and phalanges.
    • Short Bones:
    • Characteristics: Cube-like, with equal length and width.
    • Examples: Carpals (wrist bones), tarsals (ankle bones), and patella (kneecap).
    • Irregular Bones:
    • Characteristics: Bones that do not fit into the other categories due to their unique shapes.
    • Examples: Vertebrae, sphenoid bone, ethmoid bone.

Cells of Osseous Tissue

  • Types of Bone Cells:
    • Osteogenic Cells:
    • Description: Stem cells of bone tissue that give rise to osteoblasts.
    • Osteoblasts:
    • Description: Immature bone cells, non-mitotic, originating from osteogenic cells.
    • Function: Secrete the soft organic matrix of bone.
    • Osteocytes:
    • Description: Mature bone cells that have been trapped in the bone matrix; derived from osteoblasts.
    • Osteoclasts:
    • Description: Cells responsible for bone resorption (breakdown).
    • Origin: Derived from hemocytoblasts, not osteogenic cells.

Bone Matrix

  • Composition of Bone Matrix:
    • Organic Components:
    • Synthesized by osteoblasts.
    • Includes: Collagen fibers, glycosaminoglycans (GAG), proteoglycans, and glycoproteins.
    • Inorganic Components:
    • Include hydroxyapatite (a calcium phosphate salt) and various ions.
    • Properties:
    • When combined, organic and inorganic components create a strong matrix. When missing hydroxyapatite, the bone becomes soft and pliable (as seen in chicken bones).

Bone Structure: Compact vs. Spongy Bone

Compact Bone

  • Structures:
    • Osteons: The basic structural unit of compact bone.
    • Haversian (Central) Canals: Contain blood vessels (B.V.) and nerves.
    • Lacunae: Small cavities in which osteocytes reside.
    • Canaliculi: Channels that connect lacunae and facilitate communication between osteocytes.
    • Perforating (Volkmann’s) Canals: Connect Haversian canals and allow for communication between bone layers.

Spongy Bone

  • Structures:
    • Trabeculae: Slivers of bone that form a network.
    • Osteocytes Within Lacunae: Present within the trabecular network.
    • Lamellae with Canaliculi: Contain very few osteons compared to compact bone.
    • No Central Canals: Unlike compact bone, spongy bone does not have central canals.

Hormones of Osseous Tissue

  • Key Hormones:
    • Calcitonin:
    • Origin: Thyroid gland.
    • Parathyroid Hormone (PTH):
    • Origin: Parathyroid gland.
    • Calcitriol:
    • Derived from vitamin D synthesis:
      • UV light converts cholesterol to vitamin D.
      • Vitamin D travels to the liver and kidney via the bloodstream, where it is converted to calcitriol.

Effects of Hormones

  • Hypercalcemia:

    • Trigger: Release of calcitonin into the bloodstream.
    • Effects:
    • Increases osteoblast and osteocyte activity, promoting bone deposition.
    • Shuts off osteoclast activity.

  • Hypocalcemia:

    • Trigger: Release of PTH and increase in calcitriol production.
    • Effects:
    • Decreases bone deposition.
    • Increases osteoclast activity, enhancing bone resorption.
    • Increases calcium absorption from the digestive tract.
    • Reduces kidney secretion of calcium in urine.

Bone Fractures

  • Types of Bone Fractures:
    • Open (Compound): Bone breaks through the skin.
    • Closed (Simple): Bone fractures without breaking the skin.
    • Comminuted: Bone broken into three or more pieces.
    • Greenstick: An incomplete fracture where one side is broken, and the other side is bent (similar to breaking a green branch).

Fracture Repair

  • Four Main Steps in Repair:
    1. Formation of Fracture Hematoma: Initial inflammatory response with blood clots forming at the fracture site.
    2. Fibrocartilaginous Callus Formation: Granulation tissue forms, turning into a soft callus.
    3. Bony Callus Formation: Osteoblasts deposit mineral matrix and replace soft callus with hard bone.
    4. Bone Remodeling: Bone is reshaped and strengthened through various cellular processes.

Common Osseous Diseases

  • Osteomyelitis:
    • Description: Bacterial infection that causes inflammation of bone.
    • Treatment: Can now be treated with antibiotics (Rx).
  • Osteogenesis Imperfecta:
    • Description: A genetic disorder characterized by fragile bones due to defective collagen deposition.
    • Often evident at birth.
  • Osteosarcoma:
    • Description: Cancer of the bone, typically diagnosed in males aged 10-25.

Skeletal System and Ossification

  • Ossification (Osteogenesis): The process of bone formation.
  • Two Types of Ossification:
    • Intramembranous Ossification: Formation of flat bones, begins around week 8 of embryonic development.
    • Endochondral Ossification: Most bones formed through this process.

Intramembranous Ossification

  • Steps:
    1. Formation of ossification centers; mesenchymal cells differentiate into osteoblasts.
    2. Osteoblasts create trabecular structures, secreting soft matrix and depositing calcium phosphate to form spongy bone (diploe).
    3. Outermost trabeculae eventually become compact bone and periosteum.

Skull Changes After Birth

  • Fusion of Flat Bones:
    • Via sutures (fibrous connective tissue joints between skull bones).
  • Fontanels: Soft spots on the skull that close during the first year of life.
    • Fontanel Types:
    • Frontal (anterior) fontanel closes; others also include sphenoidal, mastoidal, and occipital fontanels.

Endochondral Ossification

  • Overview:
    • Begins around week 6 of development; mesenchyme transforms into hyaline cartilage, which serves as a template for ossification.

Steps for Endochondral Ossification

  • Step 1: Bone Collar Formation:
    • Triggered by blood vessel invasion of perichondrium.
    • Osteoblasts in periosteum form a bone collar around the diaphysis.
  • Step 2: Chondrocyte Hypertrophy:
    • Chondrocytes swell, leading to matrix mineralization; reduced blood supply causes cell death.
    • Deteriorated matrix acts as a splint, eventually forming primary marrow space.
  • Step 3: Periosteal Bud Formation:
    • Blood vessels, hematopoietic cells, and osteoblasts invade the cavity formed, leading to the primary ossification center.
    • Osteoblasts deposit osteoid matrix over calcified cartilage, forming spongy bone, which spreads toward the epiphyses.
  • Step 4: Medullary Cavity Formation:
    • Osteoclasts break down some spongy bone to form the medullary cavity; secondary ossification centers appear in epiphyses.
  • Step 5: Spongy Bone Development at Heads:
    • Similar process to diaphysis occurs at epiphyses due to blood vessel invasion, completing bone formation.